Horseweed, also known as marestail, is a winter or summer annual weed which reproduces by seed that germinates in spring or late summer. Seed that germinate in late summer over-winter as rosettes or basal clusters of leaves not separated by stem elongation (Figure 1). These over-wintering rosettes rapidly elongate (bolt) to produce erect flowering stems in the spring and early summer. Mature plants are unbranched at the base and may be 6 feet tall with many small flowering branches near the top as shown in Figure 2. Seeds are about 1/16 inch long with many white bristles on the end. These bristles allow for wind dispersal of the seed.

Fig. 2: Horseweed plant showing the lower part of the leafy stem, upper part of the stem with flowers, and seed with slender bristles on one end. From: Weeds of the North Central States, North Central Regional Research Publication No. 281.

Zone/No-Tillage Problem
Horseweed is native to North America and is commonly found in fallow fields, pastures, roadsides, and wasteland. Although not common in conventionally tilled and planted fields, it is common where zone/no-tillage cropping is practiced. Many of the states reporting glyphosate-resistant (GR) horseweed have a long history of no-tillage cropping. In these areas, over-wintering horseweed rosettes have likely been subjected to glyphosate selection pressure since the 1970s when growers started using Roundup (glyphosate) for burndown in no-tillage fields. With repeated glyphosate use over the years, susceptible horseweed plants were likely controlled while glyphosate tolerant plants flowered and set seed. This resulted in a shift to a horseweed population dominated by the resistant biotypes.

The International Survey of Herbicide Resistant Weeds, http://www.weedscience.org, shows that 24 states in the U.S. have documented GR horseweed populations. It appears that the increasing popularity of zone/no-tillage cropping in NY, along with the widespread use of GR crops and repeated use of glyphosate herbicides has led to the development of GR horseweed populations here as well. Greenhouse trials with horseweed seed from two locations in Central NY are being conducted to confirm this. In one case, glyphosate was applied for burndown prior to planting no-tillage soybeans and again for postemergence weed control. As can be seen in Figure 3, horseweed survived both glyphosate applications. Although field experiments to evaluate control programs for GR horseweed have not been conducted in NY State, conversations with weed science colleagues on the Delmarva, where GR horseweed was first confirmed in 2000, have been helpful in formulating control recommendations for GR horseweed in zone/no-tillage corn and soybeans. Effective control programs target the rosette stage shown in Figure 1 as part of the burndown herbicide application prior to planting zone/no-tillage crops. Once stem elongation begins, horseweed becomes increasingly difficult to control. The other key element of an effective control program is to incorporate herbicides with sites of action that are different from glyphosate, which is a Group 9 herbicide.

Burndown/Control Recommendations
The recommendations shown in Table 1 are a first attempt to make written recommendations for GR horseweed in zone/no-tillage corn and soybeans. They have not been incorporated into the Cornell Guide for Integrated Field Crop Management. They emphasize the importance of controlling horseweed early in the season when the plants are still in the rosette stage, and they incorporate herbicides with sites of action that are different from glyphosate (Group 9). For both corn and soybeans, 2,4-D LVE (synthetic auxin or growth regulator Group 4 herbicide) makes a significant contribution to horseweed burndown/control. For corn, residual herbicides like atrazine (photosynthesis inhibitor Group 5 herbicide), or Verdict a premix of Kixor (cell membrane disrupter Group 14 herbicide) and Outlook (seedling shoot inhibitor Group 15 herbicide), have proven helpful in controlling horseweed, and provide residual for control of summer annual weeds. In the soybean recommendations, OpTill or Valor XLT help control the horseweed and provide residual weed control. OpTill is a premix of Kixor and Pursuit (ALS inhibitor Group 2 herbicide), while Valor XLT combines Classic (ALS inhibitor Group 2 herbicide) with Valor, (cell membrane disrupter Group 14 herbicide).

At this time, it seems prudent that NY farmers scout zone/no-tillage acreage for horseweed that is not readily controlled with glyphosate, and that they employ an aggressive herbicide resistance management plan. Key elements of such a plan involve rotating herbicides with different sites of action, and using tank mixes/premixes or sequential applications that include herbicides with different sites of action.